The present invention relates to the making of tubing in accordance with the cold pilgrim step rolling method wherein particularly a frame moves back and forth carrying tapering rolls which roll in alternating directions upon the stock to be rolled whereby in addition a cylindrical mandrel is provided as well as turning and advancing facilities for the tube or pipe to be made and with further utilization of rotatably driven clamping and holding device downstream for extracting the tubes or pipes being made.
Cold rolling pilgrim step type rolling mills of the type to which the invention pertains and as outlined broadly above are known for some time and they have a certain inherent method of operation. This type of mill is presently used for the rolling of small thin walled tubes such as zircaloy tubes for nuclear reactor sleeves. The cold rolling in pilgrim step type and mode under utilization of a cylindrical mandrel has advantage that the ratio of wall reduction to overall diameter reduction can be quite large. This ratio is an important factor. For zircaloy sleeves when used in a nuclear reactor owing to a particular permeability for electrons. In the case of small internal diameter such as 5 mm it poses problems if conventional tapered mandrels are used in the cold, pilgrim step rolling. The quality of such mandrels leaves something to be desired and it is also difficult to position the requisite mandrel by means of the mandrel rod in rolling position.
Known mills of the type outlined above are disclosed e.g. in U.S. Pat. No. 4,090,386. Herein a long cylindrical mandrel is used and is being moved synchronously with the advance with the tube to be made in the direction of rolling. The particular mill and the commensurate rolling method being carried out have the disadvantage that any relative movement between rolled stock and mandrel is impeded. This means that additional longitudinal tension is set up in the mandrel. Owing to the high specific radial pressure that acts on the mandrel this additional tension increases the compensation tension so that the propensity of the mandrel to cold flow is increased; that is an undesired phenomenon. Broadly speaking, the danger exists that the mandrel itself is being rolled during the process.
On the other hand one could theoretically solve the problem by applying longitudinal compression on the mandrel, but that will not work since such a pressure has to be applied from the outside i.e. through the rather long mandrel rod; kinking and bend off can radially occur under such conditions particularly if the mandrels are not only rather long but also quite thin.
Practice and tests of rolling have verified that in case of unimpeded mandrel movement the velocity of the mandrel rod is smaller than the speed of the tube or pipe being rolled, but the speed of the mandrel is still larger than the advance speed of the hollow being rolled. This means that following rolling a particular length the mandrel is still stuck in the tube. One could separate mandrel and tube by using extraction devices that pull the respective tube off the mandrel. This approach requires additional structures arranged downstream of the mill for holding and clamping the tube. Moreover, this tube clamping actually means that locally the quality of the tubes or pipes being extracted may well be deteriorating. Also, additional structure is needed to move the mandrel rod and mandrel back i.e. against the direction of rolling whenever the remaining end is insufficient to reload a new hollow. Also, such a method would be time consuming and cumbersome because the long thin mandrel and rods have to be returned to a starting position outside the mill since in these cases one needs always several mandrels to work on a cyclic basis.